1. Field of the Invention
This invention relates generally to electrostatic spray guns, and more particularly relates to a removable cartridge type paint supply conduit which is used with an electrostatic spray gun to allow the gun to be used with moderately conductive as well as nonconductive paints.
2. Description of the Prior Art
Various electrostatic spray coating processes and guns have been known for a considerable time. In general, electrostatic spray coating devices have increased painting efficiency over non-electrostatic types. In principle, such systems operate as follows. Paint is atomized by a spray coating nozzle. Either prior to, at the time of, or just after atomization, the paint is electrically charged to some high voltage potential. The charging system generally operates at a potential on the order of several tens of kilovolts. In the usual situation electrostatic spray systems are used to coat electrically conductive or at least partially conductive objects. These objects are held at ground voltage potential by some appropriate means. Electrostatic forces between the charged paint and the grounded conductive object causes the paint to be drawn to the object to be coated. Because of these electrostatic forces, such phenomena as "overspray" are reduced. The electrostatic forces draw much of the atomized paint, which would otherwise have missed the object, back toward the object.
In order to charge the paint in an electrostatic spray coating system it is necessary to have some means of applying the charge to the paint. In a large majority of the systems used today charging is accomplished by an electrode connected to a high voltage power supply. The electrode is placed in close proximity to or even in contact with the stream of fluid either just prior to or very close to the point of atomization. This process works well when spraying nonconductive paints, however, when spraying paints which are moderately conductive, having for example resistivity of 200,000-1,000,000 ohm-centimeters such as metallic paint, certain precautions must be taken to prevent the high voltage at the electrode from being short circuited to ground through the column of paint.
One approach was to isolate the entire paint supply system from ground potential. This allowed the entire paint system to "float" at the charging potential. However, such an approach had several drawbacks. One of the major drawbacks was that an enormous amount of electrical energy was capacitively stored in the system. This capacitively stored energy could inadvertently be discharged in a spark, causing either an electrical shock to operating personnel, or possibly causing an explosion in an explosive environment like that which exists where solvent based paints are being sprayed.
Another approach was to ground the paint supply container, and to connect the spray gun to the container with a hose which was long enough to make the total electrical resistance of the paint column between the gun and the container large enough for moderately conductive paints so as to reduce electrical current through the paint column to a level that did not short out the electrode. However, this approach had distinct disadvantages. The paint supply hose in such systems was necessarily very bulky and hard to manage when used with a hand held gun. These hoses had to be bulky in order to provide the necessary electrical insulation, and possibly even a grounded conductive layer surrounding the hose. From an operator's point of view this approach was very burdensome.
U.S. Pat. Nos. 2,739,838 and 2,784,350 issued to Sedlacsik, German Pat. No. 2,111,271 and French Pat. No. 2,082,706 all teach various approaches to increasing the length of the paint supply hose. These devices have paint supply conduit in the form of a spiral. This allows the increased length of hose to be in a compact form. One approach mounted the spiraled coil remotely from the spray gun, and the others formed a spiral conduit either inside the barrel of a hand gun or formed a spiral conduit by wrapping a polyethylene hose around the barrel of the gun. Even the best of these approaches had serious drawbacks which prevented the design from becoming a viable commercially desirable product. In these prior art devices, almost invariably, polyethylene was used to form the spiral conduit. However, polyethylene is not impermeable to polar type solvents which are used in a large number of the moderately conductive metallic paints. Polar type solvents are solvents wherein either the molecules exhibit a separation of positive and negative charges, or wherein the centers of positive and negative charges do not coincide, for example the alcohols and ketones. These solvents are chemically active and react with the polyethylene. Because of the chemical reaction the polyethylene would tend to develop microscopic pores which would reduce or eliminate the insulating properties of the conduit. Therefore, in a hand held gun the spiral conduit of necessity was required to be placed at a distance from the operator's hand. The only practical location for the spiral was either in or around the barrel of the gun. However, this additional paint column in the barrel of the gun made the gun "front heavy" and caused severe fatigue to the operator. The spiral conduit could not have been repositioned closer to the pivot point round which the operator's hand moved since the permeability of the material used for the conduit would eventually allow the operator to be shocked if the conduit was moved any closer to his hand.
Further, the same "front heaviness" would be experienced with these prior art guns even if they were used to spray nonconductive paints. The spiral conduit could possibly have been replaced with a straight tube, but not easily. Nor could a damaged coil be replaced on the gun easily and efficiently.
Thus, no one in the past has produced a gun compatible with a spiraled conduit which was self contained, easily removed and compactly stored, and yet was not excessively front heavy and was still safe without the need for a bulky hose which had to be dragged around with great difficulty.